1. Field of the Invention
The present invention is related to a voltage converter, and more particularly, to a flyback voltage converter with high efficiency.
2. Description of the Prior Art
Voltage converters are essential devices in modern electronic devices. They can convert the input voltage of an electronic device into different operating voltages for driving the loads. Among various types, flyback voltage converters are widely used due to high efficiency, low power consumption, small size and light weight.
FIGS. 1-3 illustrate the prior art flyback voltage converters 200a-200c. The flyback voltage converters 200a-200c each include a voltage source VDC, a first electromagnetic device L1, a second electromagnetic device L2, a power switch SW, a switch control unit 210, and a rectifying circuit 230, and respectively include snubber circuits 240a-240c. The electromagnetic devices L1 and L2, which can be implemented using coupling inductors or transformers, is configured to convert an input voltage VIN into an output voltage VOUT for driving a load RLOAD. The power switch SW can be a metal oxide semiconductor (MOS) transistor which selectively switches between ON state (short-circuited) or OFF state (open-circuited) according to a control signal VGS. The rectifying circuit 230 includes a diode DO and an output capacitor COUT. When the power switch SW is turned on, the diode DO is reverse-biased and the current flowing through the first electromagnetic device L1 increases. The energy provided by the voltage source VDC is thus stored in the first electromagnetic device L1. When the power switch SW is turned off, the diode DO is forward-biased and the energy stored in the first electromagnetic device L1 is transferred or coupled to the second electromagnetic device L2 and is released to the output capacitor COUT and the load by the second electromagnetic device L2.
The switch control unit 210 includes an error amplifier 112, a pulse width modulation (PWM) comparator 113, a driving unit 114, and a feedback circuit 120. The feedback circuit 120, including a plurality of resistors in series, can provide a corresponding feedback signal VFB by voltage-dividing the output voltage VOUT. The error amplifier 112 can compare the feedback signal VFB
  (            V      FB        =                  V        OUT            ×                        R          n                                      R            1                    +                      R            2                    +          …          ⁢                                          +                      R            n                                )with a reference voltage VREF, thereby providing a corresponding control signal VC. According to the control signal VC, the PWM comparator 113 generates a corresponding control signal VPWM, according to which the driving unit 114 generates the control signal VGS. In other words, the prior art flyback voltage converters 200a-200c adjust the duty cycle of the power switch SW according to the variations in the output voltage VOUT so as to maintain the output voltage VOUT at a target level.
However in real applications, the power switch SW is not an ideal device, and leakage inductance inevitably exists in the electromagnetic devices L1 and L2. After the power switch SW switches from ON state to OFF state and before the diode DO is completely turned on, a high induced voltage is established during this period since the current of the leakage inductance rapidly drops. This can cause a quite high voltage across the power switch SW. In order to prevent the power switch SW from breaking down, the prior art flyback voltage converters 200a-200c further include snubber circuits 240a-240c, respectively.
In the prior art flyback voltage converter 200a, the snubber circuit 240a includes a resistor RS, a capacitor CS, and a diode DS. When the voltage on the power switch SW is higher than the preset voltage, the diode DS is turned on and the energy stored in the leakage inductance can be absorbed by the capacitor CS. In other words, the capacitor CS can reduce the voltage on the power switch SW. Also, when the power switch SW is turned on in subsequent stage, the diode DS is turned off and the energy stored in the capacitor CS can be discharged by the resistor RS. However, the prior art flyback voltage converter 200a has poor efficiency since the energy stored in the capacitor CS is dissipated by the resistor RS.
In the prior art flyback voltage converter 200b, the snubber circuit 240b includes two TVS (transient voltage suppressor) diodes DZ and DS. If the voltage on the power switch SW is higher than the preset voltage, the voltage on the power switch SW can be clamped by the TVS diodes DZ and DS. In other words, the snubber circuit 240b can reduce the voltage on the power switch SW. However, the prior art flyback voltage converter 200b also has poor efficiency since the energy stored in the leakage inductance cannot be recycled.
In the prior art flyback voltage converter 200c, the snubber circuit 240c includes a capacitor CS and a power switch QS. When the power switch SW is turned off, the power switch QS is turned on and the energy stored in the leakage inductance can be absorbed by the capacitor CS. By properly controlling the turn-on time of the power switch QS, the energy stored in the capacitor CS can be transmitted to the voltage source VDC and the output capacitor COUT for recycling, and the power switch SW can work at zero voltage switch-on condition. The prior art flyback voltage converter 200c can reduce switch loss of the power switch SW and improve efficiency. However, the snubber circuit 240c needs an additional power switch, and the switch control unit 210 needs two driving units 114a and 114b, which may increase manufacturing costs and circuit complexity.